2.3.3 Ethernet and IEEE 802 Encapsulation

2.3.3 Ethernet and IEEE 802 Encapsulation

The IP encapsulation for Ethernets is described in RFC-894
[LINK:3], while RFC-1042 [LINK:4] describes the IP
encapsulation for IEEE 802 networks. RFC-1042 elaborates and
replaces the discussion in Section 3.4 of [INTRO:2].

Every Internet host connected to a 10Mbps Ethernet cable:

MUST be able to send and receive packets using RFC-894
encapsulation;

SHOULD be able to receive RFC-1042 packets, intermixed
with RFC-894 packets; and

MAY be able to send packets using RFC-1042 encapsulation.

An Internet host that implements sending both the RFC-894 and
the RFC-1042 encapsulations MUST provide a configuration switch
to select which is sent, and this switch MUST default to RFC-894.

Note that the standard IP encapsulation in RFC-1042 does not
use the protocol id value (K1=6) that IEEE reserved for IP;
instead, it uses a value (K1=170) that implies an extension
(the "SNAP") which can be used to hold the Ether-Type field.
An Internet system MUST NOT send 802 packets using K1=6.

Address translation from Internet addresses to link-layer
addresses on Ethernet and IEEE 802 networks MUST be managed by
the Address Resolution Protocol (ARP).

The MTU for an Ethernet is 1500 and for 802.3 is 1492.

DISCUSSION:

The IEEE 802.3 specification provides for operation over a
10Mbps Ethernet cable, in which case Ethernet and IEEE
802.3 frames can be physically intermixed. A receiver can
distinguish Ethernet and 802.3 frames by the value of the
802.3 Length field; this two-octet field coincides in the
header with the Ether-Type field of an Ethernet frame. In
particular, the 802.3 Length field must be less than or
equal to 1500, while all valid Ether-Type values are
greater than 1500.

Another compatibility problem arises with link-layer
broadcasts. A broadcast sent with one framing will not be
seen by hosts that can receive only the other framing.

The provisions of this section were designed to provide
direct interoperation between 894-capable and 1042-capable
systems on the same cable, to the maximum extent possible.
It is intended to support the present situation where
894-only systems predominate, while providing an easy
transition to a possible future in which 1042-capable
systems become common.

Note that 894-only systems cannot interoperate directly
with 1042-only systems. If the two system types are set
up as two different logical networks on the same cable,
they can communicate only through an IP gateway.
Furthermore, it is not useful or even possible for a
dual-format host to discover automatically which format to
send, because of the problem of link-layer broadcasts.